Officials in the Russian republic of Chechnya say suicide bombers have detonated explosives in the capital Grozny, killing at least four police officers.

Authorities say the police were killed when bombers riding bicycles set off their devices in various locations in the city.

The attacks follow Monday's blast when a suicide bomber rammed his truck packed with explosives into a police station in neighboring Ingushetia, killing at least 25 people and wounding more than 130 others. Authorities say several police officers at the facility are missing and presumed dead.

Violence has increased recently in various republics of Russia's North Caucasus region, where federal troops have been fighting Islamic separatists for more than a decade.